1. Field of the Invention
The present invention relates to a charge pump and, more particularly, to a constant current output charge pump suitable for a white light emitting diode (WLED).
2. Description of Related Art
Generally, portable electronic devices (e.g. PDAs or cellular phones) require to be provided with a constant voltage, and most devices are powered by batteries. Since the output voltage may decrease as the battery being consumed for a certain time, the output voltage cannot be kept at a stable level.
For example, batteries for common cellular phones can supply an output voltage of 3.0 to 4.5V; however, the WLED for cellular phones works at a voltage higher than 3.2V-3.4V. When a battery has been used for a certain time as the voltage drops under 3.4V, it disables the WLED of the cellular phone from functioning properly. At that point, despite the battery may still be partially charged, a charge pump, according to the conventional method, shall be employed to boost the voltage up to 3.4V and above, such that the WLED of the cellular phone can continue to function until the battery is completely exhausted. However, using a charge pump to increase the power voltage contributes to a fall in the output current, causing the power supply not able to be outputted at a constant level.
Therefore, it is desirable to come up with an improved charge pump with constant current output to overcome the above flaws.